Security forces in Israel were placed on high alert Thursday, after militant groups vowed revenge for the deaths of 15 Palestinians during two separate raids by the Israel Defense Forces in the Gaza Strip the day before.

Police set up roadblocks at the entrances to Jerusalem, while public transportation security personnel were ordered to be extra watchful.

Also Thursday, the IDF announced that troops in the last month had uncovered and destroyed a Hamas laboratory for creating Qassam rockets, near the West Bank city of Ramallah. The six Hamas activists involved in running the lab were arrested, the military said.

The detainees include the man who headed the rocket-making program, Fader Tahah, 30, who was released from an Israeli jail about a year ago, Israel Radio reported.

The bomb factory contained gas canisters, a refrigerator rigged with explosives, fertilizer and rocket engines, and one Qassam rocket was in a final phase of construction, the IDF announcement said.

Security forces are concerned that the Palestinians are transferring their knowledge of Qassam rocket-making from the Gaza Strip to the West Bank, the radio said. If fired from Ramallah, the rockets could reach as far as Jerusalem.

The IDF said it was the second discovery of a rocket lab in the area. Until now, production of the rockets was limited to Gaza, from where they have repeatedly been fired at settlements in the Strip and communities in the Negev, just over the Green Line border.

Also Thursday, a Hamas activist was killed by IDF gunfire in a village west of Ramallah, Israel Radio reported. Troops entered the West Bank village to arrest Hamas activist Samer Arar, who tried to escape, according to the report. They shot and killed him when he refused to stop.

In the Gaza Strip, a Palestinian child was lightly wounded by IDF gunfire Thursday when troops detained three children - aged 7, 8 and 10 - as they tried to cross the Rafah border into Egypt. The army is investigating the suspicion that the children were sent by Palestinian organizations to find a path between Palestinian territory and Egypt, Israel Radio reported.

Before dawn, undercover Border Police officers in the West Bank city of Nablus arrested a Palestinian man they said was planning to carry out a terror attack in Israel shortly. The suspect, Ra'ad Kini, is also thought to have been involved in several shooting attacks in the Nablus region, and has been transferred to the Shin Bet security services for questioning.

Hamas warns of retaliationHamas warned that it would strike Israelis everywhere in retaliation for Wednesday's Gaza raids. All the Palestinians killed were armed or preparing explosives devices, Army Radio quoted the military as saying Thursday. Among those killed was the son of a Fatah official.

There were no Israeli casualties.

One of Hamas' leaders, Mahmoud Zahar, told Israel Radio that the military wing had urged all of its cells in Gaza and the West Bank to attack. Similar calls in the past have been followed within days - sometimes hours - by suicide bombings in Israel.

"The military wing expects that the operations will be there [Israel] at any time," Zahar said.

The military wing issued a statement calling on all its cells in the West Bank, Gaza and Jerusalem to carry out "huge martyrdom operations... everywhere in Palestine."

"We believe that these killings must stop immediately if the peace process is to go forward and bring results," Qureia said in Rome after talks with Italian Foreign Minister Franco Frattini.

During Wednesday morning's battle in Sajayieh, Izzadin al-Kassam issued a bulletin: "This is an urgent call to all our squads in the West Bank towns, villages and refugee camps. You should urgently react and hit wherever possible with large suicide bombings inside Israel and the settlements." The bulletin added: "All avenues are open for you to take revenge for the massacre at Sajayieh."

Hamas spokesman Abdel Aziz Rantisi said: "The enemy will not get away scot free and the Israelis will pay a heavy price." He expressed the hope that the revenge actions would force Israel to withdraw not only from the Gaza Strip but also "from all conquered Palestinian territories."

IDF sources said that the main purpose of the action was to root out squads that have been operating freely in the area and preparing Qassam rockets and mortars to fire against Israeli targets.

They denied that the IDF had decided on the action because of its opposition to Prime Minister Ariel Sharon's plan to evacuate settlements in the Gaza Strip.

More than 40 people were said to have been wounded during the clashes. Local radio stations kept urging civilians to leave the area. Many Gazans were infuriated that the Palestinian gunmen allowed youngsters and children to wander among them freely during the shoot-out instead of sending them home.

Colonel Yoel Strick, who commanded the IDF troops during the battle, said many dozens of armed Palestinians had participated. He also said many of them stood firing from among the women and children who had left their homes to watch the battle.

The IDF withdrew from the area at around noon and many of the Palestinians' funerals were held in the afternoon. Shortly afterward, Palestinians fired Qassam rockets at Israeli targets. By evening, 10 rockets had fallen within the Green Line and in the Gush Katif settlement bloc in the Strip. Damage was caused to one house in a settlement.

Simultaneously with the Sajiyeh action, the IDF sent troops and tanks into Rafah refugee camp. A Givati brigade special forces unit and Border Policemen carried out a search of buildings where tunnels were believed to be hidden.

The troops destroyed one building and engaged in a gun battle with armed Palestinians.

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